The present invention relates to a method of optimising the transmission of idle beacon messages in a communications system, and particularly but not exclusively, in a system having a time division duplex (TDD) frequency channel, for example in a digital cordless telephone system having time division multiple access (TDMA) protocol and also a communications system operable in accordance with the method, and to a primary and a secondary station for use in the communications system.
An example of such a communications system is the Digital European Cordless Telecommunications (DECT) system, the specification for which is currently being drafted by the European Telecommunications Standards Institute (ETSI). DECT comprises, for voice communication, one or more primary or fixed base stations which are connected by wire to the PSTN and one or more secondary or transportable, for example hand portable, stations which communicate by way of a radio link with a primary station. In a domestic environment there may be one secondary station to each primary station but in the case of a block of flats or an office environment a plurality of secondary stations will be capable of communicating via a respective primary station. In the case of DECT there are ten radio or frequency channels, and each is divided into frames of 10 milliseconds duration. Each frame is divided into 24 equal time slots (or physical channels) which comprise 12 time division duplex (TDD) channels termed duplex channels. The TDD arrangement is such that the nth and the (n+12)th time slots, where n is an integer between 1 and 12, are the forward and reverse physical channels constituting the duplex channel. Each pair of physical channels is capable of carrying one duplex digitised speech conversation or data at a rate of 1.152 Mbits/sec.
In setting up a voice call between a primary and a secondary station, a pair of physical channels is assigned to the transaction. The assignment of the pair of physical channels in any of the frequency channels is by the method of dynamic channel allocation whereby a secondary station taking into account its radio environment as determined by monitoring the average interference in each of the 120 pairs of physical channels negotiates with the primary station for access to the best pair of physical channels currently available under the control of the primary station.
There will be times, such as at night, when the base station is idle. Currently the DECT specification requires base stations to continue transmissions on their last active forward physical channel to provide the system functions of broadcasting the basic system information, base station identity and a frequency and frame timing reference. For convenience of description such transmissions will be called "normal idle beacon" transmissions. A secondary station in range of the primary station can remain tuned to the active forward physical channel but can power down. It wakes up to receive regular paging transmissions which are sent every 160 ms (or 16 frames) and to check every 2 or 5 seconds that it is correctly tuned to, and in synchronism with, the primary station. A reason given for having these normal idle beacon transmissions is that the system must be capable of setting-up a call with a secondary station already locked to a primary station within the guaranteed maximum connection time of 1.5 second specified by most telephone authorities. Therefore, a secondary station wishing to make a call must be able to scan the physical channels to obtain the best channel and then establish frame and slot synchronization using the normal idle beacon transmissions. These transmissions also enable the secondary station to save power by permitting them to power down. However, if it is quiet, these regular normal idle beacon transmissions generated every 10 ms can be a source of undesirable interference, in addition to being a waste of power. In fact some workers in the field of cordless telephones believe that base station interference in a residential area may limit the range of a secondary station because if it detects excessive interference on a channel it will search for an acceptable alternative channel. This task may be difficult if there are a relatively large number of nearby primary stations transmitting normal idle beacon signals.